1. Field of the Invention
The present invention relates to a semiconductor substrate, and to a process for production thereof. Especially the present invention relates to a substrate used for production of electronic devices, integrated circuits, and the like, and a process for production of the substrate.
More specifically, the present invention relates to a reusable reclaimed substrate which is reclaimed by working a substrate having a single crystal semiconductor layer provided on an insulating layer, and to a process for reclamation thereof.
2. Related Background Art
In a semiconductor production process, a large number of wafers are used as test wafers for monitoring film thickness in production steps and for monitoring particles, and dummy wafers in heating apparatuses. Defective wafers are unavoidably formed in the production steps and are rejected in the final inspection step.
Bulk wafers are reclaimed and reused usually for effective utilization of resources and for cost reduction of the semiconductor production. In the reclamation process, a film such as polysilicon films for LSI, insulation films, and aluminum films formed on the surface of the wafer is eliminated by etching or lapping, polishing or grinding. The mechanical elimination such as lapping is conducted to eliminate a hard film not eliminable by etching only or a complicated film (multiple layer).
As mentioned above, bulk silicon wafers having used as a process wafer, test wafers, dummy wafers, or the like are conventionally reclaimed. The process for the reclamation is described typically in Japanese Patent Application Laid-Open Nos. 9-237771 and 7122532.
On the other hand, substrates having a single crystal semiconductor layer on an insulation layer (hereinafter referred to as an xe2x80x9cSOI substratexe2x80x9d) which are produced by processing of a silicon bulk wafer through various steps are not reclaimed. Defective SOI substrates after the production, which do not meet the quality standard such as uniformity of film thickness, stacking fault density, particles, and so forth are rejected and discarded.
However, reclamation of the defective SOI substrates are demanded for repeated use in the SOI substrate production, or for wafers of other semiconductor substrates from the standpoint of cost reduction in production of SOI substrates.
The formation of single crystal Si semiconductor layer on an insulator, namely SOI substrate formation, is widely known as silicon-on-insulator (SOI) technique. The device utilizing the SOI technique has many advantages which cannot be achieved by an ordinary bulk Si substrate for Si integrated circuit, as shown below.
1. Ease of dielectric isolation and possibility for high integration,
2. High resistance against radioactive ray,
3. Low floating capacity and possibility for high speed operation,
4. Needlessness of the welling process,
5. Preventableness of latch-up, and
6. Possibility for formation of a complete depletion type of field effect transistor by thinner film formation.
These matters are described in detail in the literature: Special Issue, xe2x80x9cSingle Crystal Silicon on Non-Single-Crystal Insulatorsxe2x80x9d, edited by G. W. Cullen, Journal of Crystal Growth, vol. 63, no. 3, pp. 429-590 (1983).
The SOI structure can be formed by a process including typically an oxygen ion implantation process, and a bonding process.
The oxygen ion implantation process, called SIMOX, was firstly reported by K. Izumi. In this process, oxygen ions are implanted into an Si wafer at a density of 1017 to 1018/cm2, and thereafter, it is annealed in an argon-oxygen atmosphere at a high temperature of about 1320xc2x0 C. Consequently, Si oxide layer is formed by bonding of the implanted oxygen ions to Si around the depth corresponding to the projection range (Rp) of ion implantation. Simultaneously, above the Si oxide layer, the Si layer having turned amorphous by the oxygen ion implantation crystallizes again to change into a single crystal Si layer. Thus the SOI substrate is produced.
On the other hand, one of the bonding processes for semiconductor substrate is disclosed by Takao YONEHARA, one of the inventors of the present invention, in Japanese Patent 2,608,351 and U.S. Pat. No. 5,371,037. In this process, an SOI substrate is produced through steps of preparing a first member having a porous single crystal semiconductor layer and a nonporous single crystal semiconductor layer; bonding the above first member to a second member with interposition of an insulating layer and with the above nonporous single crystal semiconductor layer placed inside; and removing the porous single crystal semiconductor layer.
Another method of producing an SOI substrate is disclosed in Japanese Patent Application Laid-Open No. 5-211128 in which a micro-bubble layer (separation layer) is formed in a silicon base plate (first base plate) by ion implantation, the first base plate is bonded to a second base plate, the bonded article is heat-treated to cause rearrangement of crystals and coalescence of the bubbles in the bubble layer, and thereby the outermost layer (called xe2x80x9cthin semiconductor material filmxe2x80x9d in this laid-open patent) is separated at the micro-bubble layer to produce an SOI substrate. The above ion implantation is conducted with rare gas ions or hydrogen ions.
In preparation of SOI substrates by the bonding methods, the first substrate having a separation layer becomes useless after separation of the SOI structure. Yonehara, one of the inventors of the present invention, et al. disclosed a method of reusing the separated first base plate in production of a semiconductor substrate in Japanese Patent Application Laid-Open No. 7-302889.
An example of the method disclosed in the above laid-open patent publication is explained below by reference to FIGS. 10A, 10B, and 10C. A surface layer of first Si base plate 1001 is made porous to form porous layer 1002. On porous layer 1002, single crystal Si layer 1003 is formed. This single crystal Si layer 1003 is bonded to the main face of second Si base plate 1004 with interposition of insulating layer 1005 (FIG. 10A). Then the bonded wafer is separated at the porous layer (FIG. 10B). The bared porous Si layer on the surface of the second Si base plate is removed selectively to obtain an SOI substrate (FIG. 10C). The separated first Si base plate 1001 can be reused after removal of the remaining porous layer.
According to the above method, the separated first Si base plate 1001 is reusable. On the other hand, an SOI structure formed on the second Si base plate may be defective or may not meet the quality standard of film thickness distribution or particles, and so forth. In this case, the second base plate, or the SOI substrate also needs to be reclaimed. The reclamation of both the first Si base plate and the second Si base plate, if it is possible, further cost reduction can be achieved in production of SOI substrates.
In recent several-years, many reports have been presented regarding the usefulness of the SOI structure for higher speed and lower power consumption of MOSFET (IEEE SOI Conference 1994). With the SOI structure, the insulation layer under the element enables simplification of element separation process in comparison with the element formed on a bulk Si wafer, whereby the device production process can be simplified. Therefore, the SOI substrate not only improve the performance of the device, but also reduces the wafer cost and the processing cost in total. Because of the above advantages, production of SOI substrates is expected to grow more, and the reclamation of the SOI substrate will be the important problem.
The present invention intends to provide a process for reclaiming an SOI substrate.
The present invention intends also to decrease the loss of thickness of the base plate in the reclamation process and to decrease the variation of the thickness of the base plate over the surface.
The present invention further intends to provide a method of flattening of the surface of the silicon base plate after removal of the insulation layer.
The first embodiment of the present invention is a process for reclaiming an SOI substrate, comprising a step of providing an SOI substrate having a semiconductor base plate, and a single crystal semiconductor layer formed thereon with interposition of an insulation layer; a first removal step for removing the single crystal semiconductor layer; and a second removal step for removing the insulation layer selectively.
The second embodiment of the present invention is a process for reclaiming an SOI substrate, comprising a step of preparing an SOI substrate having a single crystal semiconductor layer on a semiconductor base plate with interposition of an insulation layer, a first removal step of removing the single crystal semiconductor layer by etching or polishing, and a second removal step of removing the insulation layer by etching selectively.
The third embodiment of the present invention is a process for reclaiming an SOI substrate, comprising oxidizing the single crystal semiconductor layer before the above first removal step.
The fourth embodiment of the present invention is a process for reclaiming an SOI substrate, comprising heat-treating the bared semiconductor base plate, after the above second removal step, in a reductive atmosphere containing hydrogen.
The fifth embodiment of the present invention is a process for reclaiming an SOI substrate, comprising polishing chemically and mechanically the surface of the bared semiconductor base plate after the above second removal step.
The sixth embodiment of the present invention is a process for reclaiming an SOI substrate, where the SOI substrate is prepared by a bonding method.
The seventh embodiment of the present invention is a process for reclaiming an SOI substrate, where the SOI substrate is prepared by a bonding method, and the interface between the semiconductor base plate and the insulation layer is the bonding face.
According to the present invention, the SOI substrate can be reclaimed for repeated use in the semiconductor production process. The SOI substrate can be reclaimed repeatedly with little loss of the thickness of the base plate of the SOI substrate. Therefore the reclaimed base plate can be reused for the uses in which the thickness of the base plate is limited. Therefore, the present invention is valuable from the standpoint of industrial production, resource saving, and environmental protection.
The reclaimed SOI substrate of the present invention has surface as flat as that of a bulk wafer, and is useful not only a test wafer or a dummy wafer, but also as a wafer for production of an SOI substrate repeatedly.